As tap or city water has been subjected to chlorination with chlorine gas or sodium hypochlorite, it contains residual chlorine dissolved in the form of hypochlorous ion (ClO.sup.-) or hypochlorous acid (HClO). Such residual chlorine gives rise to a smell which is commonly referred to as bleaching powder odor and is often unwelcome. Tap water also contains a small amount of organic chlorine compounds, including trihalomethane compounds such as chloroform CHCl.sub.3 and bromodichloromethane CHCl.sub.2 Br, which are produced by reaction of chlorine with organic substances. The presence of trihalomethane compounds in tap water is drawing increasing public attention as they are carcinogenic, harmful substances. Furthermore, in recent years, phytoplanktons tend to increase and propagate in water sources due to water pollution and eutrophication, so that smelly or malodorous organic substances which presumably are metabolite or secreta of phytoplanktons are present in a small content in tap water. Known in the art as such plankton-originated smelly substances are 2-methylisoborneol C.sub.11 H.sub.20 O (hereinafter, 2-MIB) and geosmin C.sub.12 H.sub.22 O. These substances are generally referred-to as "musty-smelling" substances because of their musty odor and are likewise unwelcome.
Conventionally, water purifying devices have been used for domestic or business purposes in order to remove these harmful and smelly substances from tap water to obtain healthful and palatable water. In early water purifiers, it has been customary to use granular activated charcoal which is capable of removing residual chlorine as well as, although only for a limited period of time, trihalomethanes and smelly organic substances. It is believed that residual chlorine is removed by chemical adsorption at the active sites (C--O.sup.- bond) located at the surface of activated charcoal. Accordingly, the adsorption capability of granular activated charcoal with respect to residual chlorine is deemed to be dependent on the specific surface area of activated charcoal. In contrast, trihalomethanes and smelly organic substances are believed to be physically adsorbed by activated charcoal, with the hydrated molecules thereof being trapped in the micropores of activated charcoal.
Once residual chlorine (hypochlorous ion or hypochlorous acid) is removed by bringing tap water in contact with activated charcoal, bacteria may be allowed to breed at the activated charcoal when the water purifier is out of use. As this is unhygienic, it has therefore been proposed in the art to subject the activated charcoal to sterilization by boiling it at a temperature of 100.degree.-150.degree. C. to kill bacteria (e.g., Japanese Patent Kokai Publication Nos. 49-70450 and 63-62591). Advantageously, heating of activated charcoal causes trihalomethanes to be desorbed from the micropores of activated charcoal and purged into the atmosphere, because of chloroform having a boiling point of 61.2.degree. C. and bromodichloromethane having a boiling point of 90.1.degree. C. It is considered that heating also results in dissociation of the C--O.sup.- bond at the surface of activated charcoal to revive the active sites. Accordingly, activated charcoal is regenerated in this manner with respect to trihalomethanes as well as residual chlorine so that the service life thereof is extended. However, 2-MIB and geosmin can hardly be purged by boiling because they have a molecular weight (168 and 182, respectively) larger than that of trihalomethane and, hence, have a boiling point (208.degree. C. and 254.degree. C., respectively) higher than that of water.
Apart from the possibility of sterilization and regeneration achieved by boiling, from a point of view that chlorine-originating trihalomethanes as well as plankton-originated smelly substances (mainly 2-MIB and geosmin) are to be removed thoroughly for a prolonged period of time, the adsorption capability of granular activated charcoal as performed by the "physical" adsorption by the micropores is considered insufficient for the following two reasons.
First, it is believed that, for a substance to be adsorbed by activated carbon by way of the physical adsorption process, an optimum micropore diameter exists which varies from substance to substance. However, the micropore diameter distribution of granular activated charcoal is generally unsuitable to selectively adsorb a given particular substance. To explain with reference to the graphs of FIGS. 1 and 2, the graph of FIG. 1 illustrates the cumulative micropore volume (measured by the nitrogen adsorption method and analyzed by the Cranston and Inkley method for diameter less than 150 .ANG.; measured by the mercury penetration method for diameter equal to or greater than 150 .ANG.) of three kinds of granular activated charcoal A-C available on the market. The graph of FIG. 2 shows the cumulative micropore diameter V of FIG. 1 as differentiated by the micropore diameter D to see the micropore diameter distribution of the granular activated charcoal. It will be noted from the graphs of FIGS. 1 and 2 that granular activated charcoal has a micropore diameter which is distributed over an extremely wide range. This means that granular activated charcoal is suitable to comprehensively adsorb a wide variety of substances having various particle size varying from small to large one but, as a corollary thereof, is not suitable to center the target of adsorption at a particular substance.
More specifically, to discuss the adsorption capability with respect to trihalomethanes and smelly substances, because granular activated charcoal has a substantial amount of micropores having such a pore diameter that is capable of adsorbing those substances having a particle size different from that of trihalomethanes and smelly substances, the adsorption capacity thereof available for adsorption of trihalomethanes and smelly substances is limited to the extent that it possesses additional adsorption capacity capable of adsorbing substances other than trihalomethanes and smelly substances. Accordingly, from the view point that the target is to be directed on trihalomethanes and smelly substances thereby to remove them intensively and selectively, the adsorption capability afforded by granular activated charcoal cannot be fully utilized. This means that the service life of granular activated charcoal with respect to these substances--target substances--is short.
Second problem is associated with the adsorption speed of granular activated charcoal. Although the micropore structure of granular activated charcoal is not known with any certainty, it is believed that, in view of the pore diameter distribution shown in FIG. 2, granular activated charcoal has a micropore structure as shown in the model of FIG. 3 (Ishizaki, Fibrous Activated Carbon and its Application, Chemical Engineering, July 1984, FIG. 7). As the micropores of granular activated charcoal extend in an intricate manner into the mass as shown, a substantial time is required for any substance to access the inner part of micropores. Consequently, granular activated charcoal has a slow adsorption speed. As a result, a considerable time of contact is necessary to ensure that those substances, such as trihalomethanes and smelly substances, which are present in tap water only in a small concentration are fully adsorbed.
For these reasons, water purifiers employing granular activated charcoal requires either use of a large amount of activated charcoal or frequent replacement of activated charcoal. Use of large amount of activated charcoal is unfavorable because it necessitates to increase the size of the water purifiers. As water purifiers whether of domestic or business use are used in a crowded environment such as kitchen, it is desirable that the size thereof be as small as possible.
In recent water purifiers, activated carbon fibers manufactured by carbonizing acrylic or phenolic fibers followed by activation have been increasingly used because of the improved adsorption speed thereof as compared with granular activated charcoal and due to the advantage of having a narrower pore diameter distribution. In this instance, also, typical water purifiers marketed today are designed such that cartridges of activated carbon fibers are replaced a each six months or a year, because the amount of activated carbon fibers that can be charged in compact water purifiers is similarly limited. Accordingly, periodical replacement of cartridges is still required so that a substantial running cost is incurred to purchase the carbon fiber cartridges which are highly expensive.
It has been proposed in the art to regenerate activated carbon fibers by using steam (Japanese Utility Model Kokai Publication No. 55-39095; Japanese Patent Kokai Publication No. 60-225641). However, since 2-MIB and geosmin of plankton-originated smelly substances respectively have a boiling point which is notably higher than that of water as mentioned before, regeneration of the adsorption capability with respect to these substances is not effective at all if regeneration by steam is carried out under the atmospheric pressure. Japanese Patent Kokai Publication No. 61-42394 proposes to regenerate activated carbon fibers at a temperature of 100.degree.-200.degree. C. by using pressurized or superheated steam. This method, although applicable to water treatment of an industrial scale, is difficult to apply to water purifiers for domestic or business use, because it entails a high electric power consumption and a high pressure adsorption vessel design in order to penetrate high-pressure steam superheated at such a high temperature that is capable of evaporating and purging 2-MIB and geosmin.
There has also been proposed to regenerate activated carbon fibers by heating them in air at a temperature of 350.degree.-400.degree. C. (Japanese Utility Model Kokai Publication No. 58-146595). However, as activated carbon fibers generally comprise extremely fine fibers of 5-20 micrometers in diameter so that they are readily burned and broken into pieces by thermal degradation when heated in air, it is undesirable to heat them at such a high temperature that 2-MIB and geosmin are evaporated.